1. Field of the Invention
The present invention relates to electron beam lithography and to the use of a charge dissipation layer as part of a mask forming process.
2. Description of the Related Art
Electron beam lithography may be used for producing device structures having dimensions in the sub-micron range due to the small wavelength (often less than 1 angstrom) of the electrons used. Unlike optical lithography, resolution in electron beam lithography is not presently limited by diffraction but is instead limited by electron scattering in the target materials and aberrations in the electron beam steering and shaping optics. Typical electron beam applications include mask fabrication and device fabrication by direct writing of patterns onto a resist-covered substrate. In microelectronic applications, a pattern on a substrate can be formed by first depositing a resist material on the substrate and then selectively exposing the resist material to an electron beam in a manner which locally alters a chemical or physical property of the resist. The resist material in the exposed region might, for example, either be made soluble or insoluble upon exposure to the electron beam radiation so that the solubility of the exposed region is different from that of the unexposed region. Either the exposed or the unexposed region can be removed using a solvent, depending on the particular process chosen, and the remaining resist used to protect the underlying material during processing steps such as etching or ion implantation.
One problem associated with electron beam lithography is charging of the resist by the electron beam. The resist material is typically a hardened polymer material that is highly insulating. When an electron beam is directed at the insulating surface of the resist, charge from the electron beam accumulates on the surface of the resist, creating an electric field which subsequently distorts the electron beam adjacent to the surface. Electron beams used for lithography may have a cross-section of less than 1 micron, and it is possible for even a small amount of charge to distort the electron beam to a considerably larger size or can deflect the beam by distances at least on the order of the beam diameter. The distortion results in a loss of precision and leads to pattern registration errors and poor overlay accuracy. To avoid charging the resist, a thin conducting metal layer of gold or palladium might be deposited on the resist prior to electron beam exposure. However, such a thin metal layer typically is more difficult to remove than the resist layer and additional processing steps must be added to the fabrication process when such a metal layer is used. In addition, the heat generated during the metal deposition process may degrade the lithographic properties of the resist.
U.S. Pat. No. 5,198,153 to Angelopoulos, et al., relates to a polymer material for use as a negative resist for electron beam lithography. The polymer includes a dopant species that can be rendered conductive by selective application of an energy source to the polymer. The polymer acts as a matrix into which a reagent containing the dopant species is added to form a solid solution. The dopant species is capable of disassociating upon exposure to an energy source. The dopant precursor is selected from the group consisting of onium salts, iodonium salts, borate salts, tosylate salts, triflate salts and sulfonyloxyimides. A variety of polymers including polyparaphenylevinylenes, polyanilines, polyazines, polythiophenes, poly-p-phenylene sulfides, polyfuranes, polypyrroles, polyselenophene, and polyacetylenes may be doped with the above noted precursors and then rendered conductive upon exposure to an energy source.